The present invention provides an improved process for the extrusion of liquid crystal polymers. The process produces shaped articles having enhanced mechanical properties.
Liquid crystal polymers are well-known in the art. These polymers exhibit anisotropy in the liquid phase. They may be characterized as thermotropic (i.e., liquid crystal in the melt) or lyotropic (i.e., liquid crystal in solution). Liquid crystal polymers have very stiff, rod-like molecules. In the quiescent state, the molecules line up in an ordered array in local regions, thereby forming domains. The individual domains, however, are not lined up into any particular ordered array; instead, they exhibit random orientations.
In contrast, the shaped articles which are produced by the process of the present invention exhibit enhanced mechanical properties due to the orientation of the polymer molecules parallel to the flow direction. Articles, such as films, rods, pipes, fibers, and molded articles, having the molecules oriented in such a manner are self-reinforcing, and have mechanical properties comparable to those of articles formed from fiber-reinforced isotropic polymeric materials.
According to the process of the present invention, articles having improved properties due to orientation of the polymer molecules can be formed by converting the flow of liquid crystal polymer to a substantially elongational flow, in the substantial absence of shear flow, prior to the extrusion of the polymer through a die or other extrusion orifice.
Shear flow is flow in which there is a velocity gradient transverse to the flow direction. Liquid crystal polymer domains can rotate under shear flow, thereby achieving little, if any, orientation.
Elongational flow is flow in which there is a velocity gradient parallel to the flow direction. Elongational flow can be envisioned as a stretching motion. Elongational flow can be utilized to achieve an orientation of the polymer molecules in shaped articles formed from liquid crystal polymers.
It is known in the art to provide breaker plates upstream of the extrusion orifice in extrusion apparatus. Breaker plates are commonly utilized for the purpose of preventing pressure fluctuations in the stream of fluid material in the extrusion apparatus. The cylindrical holes commonly employed in breaker plates give rise to an uncontrolled flow pattern which contains a substantial shear flow component.
Filter packs and other filter media perform the function of removing particles having a relatively large diameter from the stream of fluid material in an extrusion apparatus. Filter media produce a complex flow pattern which, again, includes a substantial shear flow component.
It is also known to those skilled in the art that the birefringence of fibers spun from liquid crystal polymers can be increased by mechanical drawing during spinning. For example, "Flow Birefringence of Polymer Liquid Crystals," Annual Report of The Research Institute for Chemical Fibers, Japan, Vol. 35, October, 1978, by Masao Horio, discloses that fibers spun from polyparaphenyleneterephthalamide, which is a lyotropic liquid crystal polymer, exhibit strong birefringence only when mechanically drawn in the spinline. For many types of shaped articles, such as sheet, films, and three-dimensional articles, however, mechanical drawing of the liquid can not readily be employed to improve the physical properties thereof. Moreover, conventional processes for extruding such articles from liquid crystal polymers, involving conditions of substantial shear flow, do not increase the birefringence of the liquid crystal polymer. Horio in fact observed no birefringence during extrusion of polyparaphenyleneterephthalamide through a glass capillary.
Therefore, it is an object of the present invention to provide an improved process for extruding liquid crystal polymer to form shaped articles which exhibit enhanced mechanical properties.
It is also an object of the present invention to provide an improved process for extruding liquid crystal polymer to form shaped articles having the polymer molecules oriented substantially parallel to the flow direction which exhibit enhanced mechanical properties.
It is also an object of the present invention to provide an improved process for extruding liquid crystal polymer to form shaped articles having the polymer molecules oriented substantially parallel to the flow direction which exhibit enhanced mechanical properties wherein the flow of liquid crystal polymer is converted to a substantially elongational flow in the substantial absence of shear flow prior to extrusion through an extrusion orifice.
It is also an object of the present invention to provide an improved process for extruding liquid crystal polymer to form shaped articles having the polymer molecules oriented substantially parallel to the flow direction which exhibit enhanced mechanical properties wherein the flow of liquid crystal polymer is converted to a substantially elongational flow in the substantial absence of shear flow prior to extrusion through an extrusion orifice by passing said fluid stream of liquid crystal polymer through at least one grid having a plurality of converging passages.